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LM4805 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4805 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 18 page  Typical Performance Characteristics (Continued) Maximum Duty Cycle vs Temperature - ”X” R DS (ON) vs Temperature 20126245 20126247 R DS (ON) vs V DD 20126248 Application Information BRIDGE CONFIGURATION EXPLANATION The Audio Amplifier portion of the LM4805 has two internal amplifiers allowing different amplifier configurations. The first amplifier’s gain is externally configurable, whereas the sec- ond amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to Ri while the second amplifier’s gain is fixed by the two internal 20k Ω resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two. This results in both amplifiers producing sig- nals identical in magnitude, but out of phase by 180˚. Con- sequently, the differential gain for the Audio Amplifier is A VD = 2 *(Rf/Ri) By driving the load differentially through outputs VO1 and VO2, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged mode operation is different from the classic single-ended amplifier configura- tion where one side of the load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration. It provides differential drive to the load, thus doubling the output swing for a specified supply voltage. Four times the output power is possible as compared to a single-ended amplifier under the same con- ditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier’s closed-loop gain without causing ex- cessive clipping, please refer to the Audio Power Amplifier Design section. The bridge configuration also creates a second advantage over single-ended amplifiers. Since the differential outputs, VO1 and VO2, are biased at half-supply, no net DC voltage exists across the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configuration. Without an output cou- pling capacitor, the half-supply bias across the load would result in both increased internal IC power dissipation and also possible loudspeaker damage. AMPLIFIER POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Since the amplifier portion of the LM4805 has two operational amplifiers, the maximum inter- www.national.com 11 |
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